1. Field of the Invention
The invention pertains generally to the manufacture of glass sheets, and, more particularly, to glass sheets formed from an overflow process.
2. Description of the Related Art
The glass that is used for semiconductor powered display applications, and particularly for TFT/LCD display devices that are widely used for computer displays, must have very high surface quality to allow the successful application of semiconductor type material. Sheet glass made using the apparatus of U.S. Pat. No. 3,338,696, assigned to Corning, Inc., makes the highest quality glass as formed and does not require post-processing. The Corning patent makes glass by a manufacturing process termed: “The Overflow Process”. Glass made using other processes requires grinding and/or polishing and thus does not have as fine a surface finish. The glass sheet must also conform to stringent thickness variation and warp specifications. The fine surface finish is formed from virgin glass primarily from the center of the glass stream. This glass has not been in contact with foreign surfaces since the stirring operation.
The teachings of U.S. Pat. No. 3,338,696 are still the state of the art as practiced today. However, the apparatus has limitations.
A major drawback of the apparatus of “The Overflow Process” is that, even though it makes excellent glass over most of the surface, the surface of the glass sheet nearest the inlet is composed of glass that has flowed in proximity to the feeding pipe surfaces and therefore is subject to lower quality.
Another drawback of the apparatus of “The Overflow Process” is that, even though its makes excellent glass during stable operating conditions, it recovers from transient conditions very slowly. This is caused in part by quiescent zones of glass flow in the pipes conducting the glass from the stirring device to the apparatus when these pipes are designed using traditional practice. During unintended process transient these quiescent zones slowly bleed glass of a previous material composition into the main process stream of glass causing defects. These defects eventually subside when the process stabilizes; however, there is a period of time where the quality of the glass sheet is substandard.
Yet another drawback of the apparatus of “The Overflow Process” is the limited means for controlling the thickness of the formed sheet. The selective cooling of the glass with respect to width as the sheet is formed is not provided in current practice.
The thickness control system of U.S. Pat. No. 3,682,609 can compensate for small thickness errors, but it can only redistribute the glass over distances on the order of 5–10 cm.
Another drawback of the apparatus of “The Overflow Process” is that the forming apparatus deforms during a manufacturing campaign in a manner such that the glass sheet no longer meets the thickness specification. This is a primary cause for premature termination of the production run.
A further drawback of the apparatus of “The Overflow Process” is that surface tension and body forces have a major effect on the molten glass flow down the external sides of the forming apparatus causing the sheet to be narrower than the forming apparatus and the edges of the formed sheet to have thick beads.
U.S. Pat. No. 3,451,798 provides for edge directors which endeavor to compensate for the surface tension effects but are in reality a correction for problems created by restricting the forming apparatus cross-section to a single profile on its external surface.
Another drawback of the prior art is that the glass sheet is not inherently flat when drawn from the forming apparatus.
Yet another drawback of the apparatus of “The Overflow Process” is that the drawing of the sheet from the bottom of the apparatus has a propensity to have a cyclic variation in sheet thickness. This cyclic thickness variation is a strong function of uncontrolled air currents, which tend to become more prevalent as the equipment ages during a production campaign. As the apparatus ages, air leaks develop through cracks in material and assorted openings caused by differential expansion.
Therefore, there is a need in the art for an apparatus which overcomes the shortcomings of the prior art.